Chymotrypsin



Chymotrypsin (Chy or alpha Chy) is a digestive enzyme containing an active serine residue. It cleaves peptide bonds of proteins where the amide side of the bond is an aromatic amino acid like tyrosine, phenylalanine or tryptophane. The Chy precursor is the inactive chymotrypsinogen (Chygen) which gets cleaved 4 times by trypsine losing a 6 amino acid long peptide to become the active Chy. Gamma Chy is a covalent acyl adduct of alpha Chy. Delta Chy results when Chygen is cleaved only twice by trypsin. The images at the left and at the right correspond to one representative Chymotrypsin, i.e. the crystal structure of Cellulomonas Bogoriensis Chymotrypsin (2ea3).

Overview
Please click on the green links as you read through the text and watch how the 3D picture on the right changes. Chymotrypsin is a protease, an enzyme catalyzing the hydrolysis of peptide bonds of proteins. Chymotrypsin helps to digest proteins in our food. Other proteases are crucial for blood clotting (thrombin and other proteases), for the AIDS virus metabolism (HIV protease) and for many other processes relevant to human health and agriculture.

While chymotrypsin occurs in many organisms, the most-studied chymotrypsin is that from cows (bovine chymotrypsin). In its mature form, bovine chymotrypsin is a protein consisting of 245 amino acids. This string of amino acids folds into a compact structure. (Can you guess where the substrate might bind? Try spinning around the molecule by dragging it with the mouse cursor. There should be a pocket somewhere on the surface of the enzyme. The active site is colored in this hint ). The path of the backbone is easier to see in this backbone cartoon, which shows that chymotrypsin folds into two large beta sheets.

Active site residues
The active site of an enzyme is the location where the substrate binds and where the chemical reaction occurs. Active site residues are those amino acid residues demonstrated to have importance for catalysis or substrate binding. Chymotrypsin contains three residues, Ser 195, His 57 and Asp 102, which are known as its catalytic triad. Similar three-dimensional arrangements of a serine, a histidine and an aspartate are observed in many other proteases, and the role of these three residues in catalysis has been studied extensively. Serine acts as a nucleophile (contributing the electron pair for a new bond) attacking the carbonyl carbon of the peptide bond to be hydrolyzed. Histidine and aspartate turn serine into a better nucleophile by assisting in removing a hydrogen ion from serine.

3D Structures of Chymotrypsin
Update June 2011

Native Chymotrypsin
1yph – BtChyA chain A - Bos taurus

4cha, 5cha – BtChy

2jet – RnChygen B chain A,B - Rattus norvegicus

1kdq – RnChyB, chain B (mutant)

2ea3 – Chy – Cellulomonas bogoriensis

1ab9, 8gch, 1gct, 2gct, 3gct, 2gch - gamma BtChy

Native Chymotrypsinogen
2cga, 1chg – BtChygen A

Chymotrypsin + polypeptide inhibitors
1cbw, 1mtn - BtChy+BPTI

1t7c, 1t8l, 1t8m, 1t8n, 1t8o – BtChyA+P1 BPTI variants

1oxg – BtChyA+autolysis peptide

1p2m, 1p2n, 1p2o, 1p2q – BtChyA+ 4 amino acids in S1 pocket

1n8o – BtChyA+ecotin

1ca0 – BtChy+APPI

1acb – BtChy+Elgin C

2cho – BtChy+turkey ovomucoid third domain

Chymotrypsin + inhibitors
3bg4 – ChyA chain A+guamerin

2p8o - BtChyA chain A+benzohydroxamic acid/vanadate

1eq9 – Chy+PMSF – fire ant

2cha –BtChy+p-sulfinotoluene

Gamma Chymotrypsin + inhibitors
1gg6 – gamma BtChy+N-acetyl-phenylalanine trifluoromethyl ketone

1ggd – gamma BtChy+N-acetyl-phenylalanine trifluoromethyl aldehyde

1afq - gamma BtChy+synthetic inhibitor

3gch, 4gch, 5gch - gamma BtChy+cinnamate

6gch, 7gch - gamma BtChy+trifluoromethy ketone

6cha – gamma BtChy+phenylethane boronic acid – transition state inhibitor

1gmc, 1gmd – gamma BtChy+hexane – transition state inhibitor

2gmt - gamma BtChy+N-acetyl-alanyl-phenylalanyl-chloroethyl ketone 1gmh, 1gcd - gamma BtChy+organophosphoryl

1gha, 1ghb - gamma BtChy+ N-acetyl-tryptophan

Gamma Chymotrypsin + reaction transition state inhibitors
6cha – gamma BtChy+phenylethane boronic acid

1gmc, 1gmd – gamma BtChy+hexane

Delta Chymotrypsin + inhibitors
1dlk – delta BtChy+peptidyl chloromethyl ketone

Chymotrypsinogen + inhibitors
1gl0, 1gli – ChygenA+PMP_D2v – Locusta migratoria

1k2i - BtChygen+7-hydroxycoumarin

2y6t – BtChygenA + ecotin